1. Field of the Invention
The present invention relates to a wavelength-division multiplexing passive optical network, and more particularly to a passive optical network including a signal monitoring unit for compensating an optical signal when a wavelength band of the optical signal changes.
2. Description of the Related Art
A wavelength-division multiplexing (WDM) scheme, which is adaptable for a passive optical network (PON), assigns each subscriber unit with an intrinsic wavelength. The PON includes a central office for generating a plurality of downstream optical signals provided to subscriber units and detecting upstream optical signals, a plurality of subscriber units for detecting downstream optical signals and generating upstream optical signals, and a remote node for relaying optical signals between the central office and the subscriber units.
The PON usually has a typical double star-type network structure in which the central office is linked to the remote node through a single optical path, and the subscriber units are linked to the remote node. Further, the PON includes a plurality of optical devices. In particular, the optical paths for linking the central office with the remote node and for linking the remote node with the subscriber units are constructed using optical fiber including silica. Typical optical devices have refractive indices varied depending on the change of a temperature, and these varied refractive indices are a factor of changing wavelength bands of downstream and upstream optical signals transmitted through corresponding optical devices.
The change of wavelength bands of downstream and upstream optical signals as described above deteriorates an optical signal receiving efficiency in the central office or each subscriber unit receiving the optical signals. In addition, the change of the wavelength bands may cause a variety of erroneous operations of a system. For example, the change of the wavelength bands may make it difficult to detect the received state of the downstream and upstream optical signals.
In order to solve the problems described above, a variety of methods and apparatus for monitoring the change of wavelength bands of optical signals according to the change of a temperature and compensating the optical signals have been suggested.
In U.S. Pat. Registration No. 6,304,350 (“Temperature compensated multi-channel, wavelength-division multiplexing passive optical network” issued by Doerr), a wavelength-division multiplexer/de-multiplexer must include additional ports for monitor light. In Doerr, the monitor light is transmitted/received through an optical path, and the wavelength bands of optical signals are monitored based on the degree of change in the intensity of the monitor light.
Thus, the conventional PON must assign additional ports to a multiplexer/de-multiplexer in order to monitor the wavelength change of optical signals. This degrades the communication efficiency of the PON and makes it difficult to maintain the PON.